The present invention relates to a device for, very rapidly and with a very high precision (some tenth of micrometers), being able to position, relatively heavy carriages (several kilograms) along a relatively long path, (more than 1 m).
Such movements are required in all possible machining tools and robots. This requirement is naturally greater in machinery where a large number of operative steps, which each takes a short time, shall be performed for positions which are maintained very exactly. An example are picking robots, where a picking head is arranged to fetch various types of components from a large number of possible places and deliver these to a large number of different places with a high precision.
The conventional art for such displacements is a screw and a belt.
Screw driven systems (see FIG. 1) use a stationary motor 101, a rotatable but otherwise stationary (ball) screw 102, which is driven by said motor 101, and a (ball) nut 103, which when said screw is rotated runs along said (ball) screw. Screw driven systems have no large problems for low speeds and short displacements. For high speeds and long displacements there are three cooperating disadvantages:
1) The critical number of revolutions: Because said screw is only supported in its ends, it will be unstable, if the number of revolutions will exceed a certain value. This value can be increased if the diameter is increased.
2) The pitch: the greater displacement the nut will do along said screw for each revolution, the lower number of revolutions is required for a certain speed. The pitches are, becayse of practical reasons, at most equal to the diameter of the screw, whereby a large pitch will give a large diameter.
3) The moment of inertia: For large diameters and long screws the moment of inertia of the screw will be an obstacle to obtaining high rotational speeds.
Belt driven systems (see FIG. 2) have a belt 201 in a closed loop about two rollers 202 and 203 and a carriage 204 running on a separate linear guide 205. The driving is accomplished through a motor 206. By activation of the motor 206, the axis of the motor 206 and the pulley 203 can be rotated. An angular movement of the axis of the motor 206 will cause a linear movement of the carriage.
Belt driven systems are limited by the elasticity of the belt. When the motor 206 produces a torque, the belt 201 will be extended, which will give an elastic offset between the angular position of the axis of said motor 206 and the position of the load/carriage 204. If the elasticity of the connection between the motor 206 and the carriage 204 was negligible, a change of the torque provided by the motor 206 would immediately create a corresponding change of the force acting at the carriage 204. However, due to the elasticity of the belt 201, a change of the torque provided by the motor 206 will not immediately influence the carriage 204 because the belt will initially elastically elongate before a force acting on the carriage 204 will displace it. Due to this and to the accompanying elastic swinging movements the time required to a obtain a precise position of the carriage will be substantially increased.
Another problem of belt drives is friction/slip. If the belt is a toothed belt the biasing force required to reduce the elasticity because of loop formation will produce large friction; if the belt is a steel belt a slip may be produced between the belt and the rollers 202/203, which will produce an offset between the angular position of the axis of said motor 206 and the position of the load/carriage 204, this offset meaning that that there will not a direct correspondence of the angular position of the motor axis to the position of the carriage.
A possible method which however, as far as is known, seldom is used for the combination of a high speed and a high precision is a stationary gear rack and a motor located on the carriage according to FIG. 3. A carriage 301 runs along a linear guide 302 arranged in some way, in which there is a stationary gear rack 303. On said carriage 301 there is a motor 304 having a gear wheel 305. To move the carriage 301 the motor shaft is rotated, which through said gear wheel 305 will force a movement of said carriage 301. The elasticity may be made neglectable because the motor shaft and the gear wheel 305 are extremely more strong and shorter than said belt 201. The gear rack 303 can be attached to the frame of the apparatus (not drawn in order not to obscure the other elements) at very short distances. A slip may not occur. The moment of inertia is known and significantly much smaller than for large, long ball screws.
An apparent disadvantage in this method is that the mass of the driving motor is added to the mass of the carriage. If high accelerations and speeds are required the driving motor will form a dominating part of the mass of the carriage. This will increase the power requirement and will entail increased costs by a need for a stronger guiding path and a stronger frame. (The frame must be constructed in such a way that it is able to resist the acceleration forces without unacceptable vibrations.) Another problem in this method is that the play between a gear rack and a gear wheel must be eliminated for the final positioning. This may be performed with double biased gear wheels. In order to operate with large torques required for a rapid positioning this will give high surface pressures and a rapid wear.
A device for the transport and positioning of work pieces is previously known from the embodiment shown in FIG. 6 and the associated part of the description in CH A 5 663 171. In this prior device carriages are displaced carrying work pieces on a path by means of exterior friction rollers. These friction rollers are rotatably mounted to the frame of said machinery. For the final and fine positioning of said carriages a gear rack segment is used placed on said carriage and an exterior gear wheel which is rotatably mounted to the frame of the machinery. The friction rollers and the gear wheels are motor driven. In such a system not very large running speeds or high positioning retardations can be achieved.
The problems in the prior methods and devices described above are eliminated by the invention.